At the top of any page you have open you'll see a web address.
If you highlight this line you can copy and paste it into a thread as a line of text.
It will appear like this: www.abovetopsecret.com...
You can also use the bb code [url=], but the brackets that look like this ], then you can enter anything you want for the link.
It will appear like this: How To Post A Link
If you click the quote button on my post you can see the code I used.